Sarcomeres, the functional units of contraction in striated muscle, are composed of an array of interdigitating protein filaments. Direct interaction between overlapping filaments generates muscular force, which produces animal movement. When filament length is known, sarcomere length successfully predicts potential force, even in whole muscles that contain billions of sarcomere units. Inability to perform in vivo sarcomere measurements with sub-micrometer resolution is a long-standing challenge in the muscle physiology field and has hampered studies of normal muscle function, adaptation, injury, aging, and disease, particularly in humans.
Laser diffraction, biopsy clamps, and two-photon microscopy have been used in prior attempts to measure in vivo sarcomere length, but each has limitations. Laser diffraction and biopsy clamps are restricted to surgical settings, limiting access to muscle types and patients available for study. Two-photon microscopy is an emerging technique which measures sarcomere length through a microendoscopic probe. This type of procedure allows for greater access to muscles and patient populations. Two-photon microscopy, however, is a point scanning method which by its nature struggles with acquisition speed, measurement depth, and motion artifacts.
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